Beta lymphocyte maturation is characterized by a series of phenotypic changes which generally reflect genetic modifications. However, there are some genetic alterations, such as the aberrant rearrangements of non-expressed immunoglobulin genes observed in some myelomas, which are not phenotypically apparent. It is clear that the genetic processes which occur during development are more complex than we have had tools to analyze in the past. Now that recombinant DNA technology provides a means of analyzing at the molecular level events which are observed at the phenotypic level, it is essential to do the analysis in a system in which cells at different developmental stages can be positively identified and isolated. I propose to study developmental processes at the genetic level in the chicken, an animal which offers advantages for such investigation because it has a completely compartmentalized immune system. The cellular conditions that exist at specific times of specific developmental stages have been characterized in the chicken; this will allow me to study normal populations of cells before and after defined developmental processes. In addition, several laboratories have successfully transformed cells of the Beta lymphoid lineage at several developmental stages using avian retroviruses. The use of these transformation systems will enable me to study alterations in the pattern of genetic development which influence or are influenced by viral transformation. These studies should elucidate some mechanisms of the regulation of gene expression in development.